1. Field of the Invention
This invention relates to a receiving apparatus for receiving digital audio broadcasting.
2. Description of Related Art
DAB (Digital Audio Broadcasting) has been known as digital communication using phase modulation. The DAB is practically used according to EUREKA 147 standard in Europe, the signal processing on the transmission side is described herein under.
(1) A digital audio data having the maximum of 64 channels is compressed according to the MPEG audio layer II for every channel.
(2) Each channel data resulted from the above-mentioned (1) is subjected to error correction encode processing by convolution coding and interleaving of the time axis.
(3) The result of the above-mentioned (2) is multiplexed to one channel. Auxiliary data such as PAD (Program Associated Data) is added.
(4 ) The result of the above-mentioned (3) is subjected to interleave processing on the frequency axis and a symbol for synchronization is added simultaneously.
(5) The result of the above-mentioned (4) is subjected to OFDM processing (Orthogonal Frequency Division Multiplex) and subsequently subjected to D/A conversion.
(6) The main carrier signal is subjected to QPSK modulation (Quadrature Phase Shift Keying) depending on the result of the above-mentioned (5), and the QPSK signal is transmitted.
The DAB is broadcast using the first frequency band and the second frequency band , which are different in frequency each other, namely VHF band (frequency band range from 174.928 to 239.2 MHz) and L band (frequency band range from 1452.96 to 1490.624 MHz).
Therefore, a DAB receiving apparatus may have the structure, for example, as shown in FIG. 1.
In FIG. 1, when a DAB of VHF band, namely the first frequency band, is received, the broadcast wave signal is received by an antenna 11, the received signal is supplied to a first mixer circuit 15 through a signal line comprising a tuning circuit 12, a high frequency amplifier 13, and a tuning circuit 14 arranged in this order, and a local oscillation signal S16 is supplied from a local oscillation circuit 16 to the mixer circuit 15.
In this case, the tuning frequency of tuning circuits 12 and 14 is changed in a frequency range from 174.928 to 239.2 MHz with tuning operation by a listener. The frequency f16 of the local oscillation signal S16 is changed in a frequency range from 136.016 to 200.288 MHz with interlocking to the tuning frequency of tuning circuits 12 and 14.
Accordingly, in the mixer circuit 15, a broadcast wave signal S12 (frequency of f12) which a listener wants to receive is frequency-down-converted to a first intermediate frequency signal S15 having a frequency of f15, namely 337.408 MHz according to the equations described herein under.
f15=f12-f16
=(174.928 to 239.2 MHz)-(136.016 to 200.288 MHz)
=38.912 MHz
The intermediate frequency signal S15 is supplied to a mixer circuit 18 through a band pass filter for intermediate frequency filter 17, a local oscillation signal S19 having a predetermined frequency of f19 (=36.864 MHz) is generated in a local oscillation circuit 19, and the local oscillation signal S19 is supplied to the mixer circuit 18.
As described herein above, the intermediate frequency signal S15 is frequency-down-converted to an intermediate frequency signal S18 having frequency f18 of 2.048 MHz according to the equations described herein under.
f18=f15-f19
=38.912 MHz-36.864 MHz
=2.048 MHz
The intermediate frequency signal S18 is supplied to an A/D converter circuit 23 through a band pass filter for intermediate frequency filter 21 and an amplifier for intermediate frequency amplification 22 for A/D conversion to a digital intermediate frequency signal, the signal is supplied to an orthogonal demodulation circuit 24 and I-component data and Q-component data are digital-demodulated.
Further, these data are supplied to an FFT (Fast Fourier Transform) circuit 25 for OFDM demodulation, the OFDM demodulated data are supplied to a Viterbi decoder circuit 26 for deinterleaving and error correction, and a program (channel) is selected in the Viterbi decoder circuit 26 and thus a digital audio data of the desired program is selected.
Subsequently, the selected data is supplied to a data expansion circuit 27 for performing MPEG data expansion, and a digital audio data expanded to the original data length is outputted from the data expansion circuit 27, the outputted digital audio data is supplied to a D/A converter circuit 28 for D/A conversion to an analog audio signal, and the signal is outputted to terminals 29.
On the other hand, when L band DAB, namely the second frequency band, is received, the broadcast wave signal S32 is received by an antenna 31, the received signal S32 is supplied to a mixer circuit 35 through a signal line comprising a band pass filter 32, a high frequency amplifier 33, and a band pass filter 34 arranged in this order, and a local oscillation signal S36 having a predetermined frequency f36 is supplied from a local oscillation circuit 36 to the mixer circuit 35.
In this case, the band pass filters 32 and 34 have a passing L band frequency range from 1452.96 to 1490.624 MHz. The frequency f36 of the local oscillation signal S36 remains 1251.424 MHz regardless of the received frequency.
In the mixer circuit 35, the L band broadcast wave signal S32 is frequency-converted to an intermediate frequency signal S35 having a frequency f35 by down-conversion according to the equations described herein under.
f35=f32-f36
=(1452.96 to 1490.624 MHz)-1251.424 MHz
=201.536 to 239.2 MHz
The intermediate frequency signal S35 is supplied to the tuning circuit 14 through a band pass filter for intermediate frequency filtration 37 and an amplifier for intermediate frequency amplification 38. In this case, the tuning frequency f14 of the tuning circuit 14 is changed correspondingly to a broadcast wave signal which a listener wants to receive out of L band broadcast wave signals, and the oscillation frequency f16 of the local oscillation circuit 16 is changed in a frequency range from 162.624 to 200.288 MHz correspondingly to the tuning frequency f14 of the tuning circuit 14.
Accordingly, in the mixer circuit 15, the intermediate frequency signal S14 of a broadcast wave signal which a listener want to receive is frequency-converted to an intermediate frequency signal S15 having a frequency f15 according to the equations described herein under.
f15=f14-f16
=(201.536 to 239.2 MHz)-(162.624 to 200.288 MHz)
=38.912 MHz
The intermediate frequency signal S15 is processed in the band pass filter 17 and subsequent circuits in the same manner as described for the VHF band processing, and L band DAB audio signal is outputted to terminals 29.
The outline of a receiving apparatus which receives VHF band and L band DAB is described herein above.
FIG. 2 shows a detailed example of local oscillation circuits 16, 19, and 36, the example comprises a PLL, further in this example, a variable frequency division circuit of the PLL comprises a pulse swallow type counter.
In detail, the pulse swallow type counter 42 comprises a prescaler 42P, a main counter 42M, and swallow counter 42S. An oscillation signal of the VCO 41 is divided into a predetermined frequency by the prescaler 42P and main counter 42M, and then supplied to a phase comparison circuit 43. A reference frequency signal is supplied from a generation circuit 44 to the comparison circuit 43. A comparison output of the comparison circuit 43 is supplied to the VCO 41 through a loop filter 45 as a control signal.
Further, when the PLL is used as the local oscillation circuit 16, the frequency division ratio of the main counter 42M is changed correspondingly to the received frequency, and on the other hand, when the PLL is used as the local oscillation circuits 19 and 36, the frequency division ratio of the main counter is fixed. An oscillation signal of the VCO 41 is supplied to mixer circuits 15 and 18, or 35 as local oscillation signals S16 and S19, or S36.
A receiving apparatus for receiving VHF band and L band DAB is structured as described herein above, the above-mentioned receiving apparatus requires, for example, three PLLs as described in FIG. 2 as local oscillation circuits 16, 19, and 36 to result in increased circuit scale, and the requirement prevents a receiving apparatus to be made small-sized and leads to increased power consumption on the same reason.
Further as shown in FIG. 3, in VHF band, the frequency of the received signal S12 ranges from 174.928 to 239.2 MHz, and the frequency of the local oscillation signal S16 ranges from 136.016 to 200 288 MHz. Because the received signal S12 is down converted to the intermediate frequency signal S15 using the local oscillation signal S16, the image frequency fimg is changed to range from 97.104 to 161.376 MHz, the change brings the image frequency fimg closer to the frequency band of the received signal S12. Therefore, the image disturbance removal performance becomes poor unless tuning circuits 12 and 14 having high Q are used to obtain sufficient attenuation performance for the image frequency fimg.
However, to have high Q, a coil and variable capacity diode having high Q are required to structure tuning circuits 12 and 14, or alternatively tuning circuits 12 and 14 having a multi-stage structure or double tuning circuits are required. These structures prevent a receiving apparatus to be made small-sized, and leads to high cost.
The present invention is accomplished to solve problems described herein above.
Accordingly, L band and VHF band received signal is frequency-converted to a first intermediate frequency signal using an oscillation signal of the first, namely the front end, local oscillation circuit.